Can't convert the Ruler/Protractor into any Geometry type
This isn't a bug. Once drawn ruler gets saved into a Grease Pencil layer specifically created for it. However this layer will be hidden so that's why it seems that ruler disappears. With it visible converting a ruler into geometry is the same as for converting GP strokes into curves.  

After hitting Enter to store the ruler, open Properties shelf with N, scroll it to the Grease Pencil layers rollout and enable visibility for the layer with the ruler/-s (it should be called something like "RulerData3D"). Make sure to select the layer itself. Then options in the Toolshelf > Grease Pencil > Convert to Geometry will become active.



Note that as Store ruler is used after drawning it to pass it to Grease Pencil and then - to geometry, if you press Ruler/Protractor to draw another one, the one you drew before will be still there (it is saved as ruler regardless).